1. Field of the Invention
The present invention relates to a vaporizer for MOCVD and a method of vaporizing raw material solutions for MOCVD.
2. Description of the Related Arts
Problematic in the development of DRAMs is a reduction in storage capacitance resulting from the miniaturization. Any measures are needed since the capacitance has to be level with that in the precedent generation from the viewpoint of software errors or the like. As a measure for this, increase in the capacitor area has been aimed at by introducing a three-dimensional structure referred to as a stack structure or a trench structure for cell structures exceeding 4M in addition to the planer structure for 1M or less. A dielectric film has also been employed which consists of a thermal oxide film and a CVD nitride film laminated on the poly-Si from the thermal oxide film of the substrate Si (this laminated film is referred to commonly as an ON film). For 16M DRAM, in order to further increase the area contributing to the capacitance, there have been introduced stack types such as a thick-film type making use of the side or a fin type utilizing the back of the plate as well.
Such three-dimensional structures have disadvantageously given rise to an increase in the stages due to the complicated process and a reduction in the yield due to the increased steps, rendering the realization in 256 Mbits or larger DRAMs difficult. For this reason, conceived as one way to further increase the integration degree without altering the current DRAM structures was a switching of the capacitance dielectrics to ferroelectrics having a higher dielectric constant. First attention was paid as a dielectric thin film having a high dielectric constant to a thin film of high-dielectric-constant single-metal paraelectric oxides such as Ta2O5, Y2O3 and HfO2. The relative dielectric constants of Ta2O5, Y2O3 and HfO2 are of the order of 28, 16, 24, respectively, which are four to seven times that of SiO2.
Nevertheless, application to 256M or larger DRAMs necessitates a three-dimensional capacitor structure. (BaxSr1-x)TiO3, Pb(ZryTi1-y)O3 and (PbaL1-a) (ZrbTi1-b)O3 are promising as materials having a higher relative dielectric constant than the above oxides and expected to be applicable to DRAMS. Similarly promising is a Bi-based laminar ferroelectric material having a crystal structure extremely resembling the superconductor. From the viewpoint of low-tension drive and good fatigue characteristics, remarkable attention is being paid recently to SrBi2TaO9 referred to as Yl material in particular.
The formation of SrBi2TaO9 ferroelectric thin film is typically carried out by means of practical and promising MOCVD (metal organic chemical vapor deposition).
Raw materials of the ferroelectric thin film are typically three different organometallic complexes, Sr (DPM)2, Bi(C6H5)3 and Ta (OC2H5)5, which are each dissolved in THF (tetrahydrofuran) solvent for use as a solution. DPM is an abbreviation of dipivaloylmethane.
Table 1 shows the respective material characteristics.
TABLE 1BOILING POINT (° C.)/PRESSURE (mmHg)MELTING POINT (° C.)Sr (DPM)2242/14  78Bi (C6H5)3270~280/1201Ta (OC2H5)5146/0.1522THF67−109
The system for use in MOCVD comprises a reacting section for subjecting SrBi2TaO9 thin film raw material to a gas phase reaction and a surface reaction for film deposition, a feeding section for feeding SrBi2TaO9 thin film raw material and an oxidizing agent to the reacting section, and a collecting section for collecting reaction products obtained in the reacting section.
The feeding section is provided with a vaporizer for vaporizing the thin film raw material.
Known techniques related to the vaporizer are illustrated in FIG. 12. FIG. 12A shows a so-called metal filter method in which a raw material solution heated to a predetermined temperature is drip fed to a metal filter used with the aim of increasing the area of contact between the ambient gas and the SrBi2TaO9 ferroelectric thin film raw material solution.
This technique however has a deficiency that the metal filter may become clogged after several-times vaporizations, making the long-term use difficult.
FIG. 12B depicts a technique in which 30 kgf/cm2 of pressure is applied to a raw material solution so as to allow the raw material solution to be emitted through 10 μm pores and expanded for evaporation,
This technique however entails a problem that the pores may become clogged as a result of several-times operations, rendering it difficult to endure the long-term use.
In the event that the raw material solution is a mixture solution of a plurality of organometallic complexes, e.g., Sr(DPM)2/THF, Bi(C6H5)3/THF and Ta(OC2H5)5/THF and that this mixture solution is vaporized by heating, the solvent (THF in this case) having a highest vapor pressure will be vaporized earlier, with the result that the organometallic complexes may be deposited and adhered onto the heated surfaces, blocking a stable feed of raw materials to the reacting section.
Furthermore, it is demanded in order to obtain a film having a good uniformity by MOCVD that there should be presented a vaporized gas within which the raw material solutions have uniformly been dispersed. However, the above prior art has not necessarily met such a demand.